I. Field of the Invention
This invention relates to propulsion systems for aircraft and, in particular, to a turbocharged internal combustion engine compounded with a gas turbine to drive a ducted fan to generate the required propulsive thrust for most efficient operation at high subsonic flight speeds through the upper limits of the tropopause into the lower levels of the stratosphere.
II. Description of the Prior Art
It has been previously recognized that internal combustion engines are more fuel efficient than gas turbine engines in aircraft. The specific fuel consumption of a conventional gas turbine increases with altitude and is also higher at part load as compared to the internal combustion engine. Well designed turbocharged piston engines are capable of providing a specific fuel consumption at high altitudes equal to or better than that available at sea level, with part load specific fuel consumption also equal to or better than that attained at maximum load.
However, gas turbines have largely replaced internal combustion engines for low altitude operation due to their smaller size and lighter weight for a given thrust level. Although this size and weight advantage diminishes with ascending altitude, gas turbines have been the powerplant normally used for high altitude high speed flight due to the absence of a more efficient, lighter weight option.
It has also been recognized that propellers or propulsors which moderately accelerate a large volume of air are highly efficient at flight speeds of about mach 0.6 or less, while turbo fans which greatly accelerate a smaller volume of air are less efficient at such low air speeds, but increase in efficiency as flight speed increases. Therefore, for high subsonic or supersonic flight, turbo fans or turbo jets have previously been the preferred propulsion systems.